I honestly think slavery was the majority cause of the Civil War. I'm a history minor and have taken multiple classes that cover this topic including History 333 which is the study of the Civil War.
Slavery was a state right, but when the nation began to expand out west and territories were created, heavy tension arose over the argument whether these territories should have discretion on the use of slavery. A majority of the political leaders in the North were industrialists and wanted these territories to turn into free states that used free labor and paid wages. (A key component to remember though is that Northerners were not opposed to slavery due to religious beliefs or morals, they just wanted to make their pockets fatter and control the economy)
On the other hand Southerners only knew how to run their economy and trade through slavery. They felt that territories should have the right to decide whether they be free or slave. This is where tension became very high and events such as Bleeding Kansas and the Missouri Compromise began to unfold and lead to the Civil War. The South was deeply rooted in a tradition of slavery and felt their way of life was threatened. (This was very unfair to African-Americans and in no way a justifiable reason to start a war, but unfortunately it was the way of life then)